In heat recovery systems the two fluids are usually gases, the temperature difference is not large and the allowable pressure drop is small. These conditions usually lead to the requirement of a large heat exchange surface. In addition, since the gases are usually corrosive, poisonous or explosive when mixed, the heat exchanger must present good corrosion resistance and good sealing of the two streams. Also, since the quality of the heat recovered is usually low, the heat exchanger must be sufficiently inexpensive to justify the cost of the investment. These conflicting requirements are not always met by existing heat exchangers.
Several types of heat exchangers are currently being employed for heat recovery. One such type is the regenerative heat wheel formed by a wheel of high thermal capacity which rotates and transports heat between the two streams. This type presents the severe disadvantage of leakage between the two streams and to the environment, and may result in appreciable loss of pumping power. Leakage may preclude application when the admixture of the two gases may cause fires or when the gases are poisonous. Another heat exchanger utilizes cast finned tubes. These exchangers are heavy and bulky, and present low resistance to both low and high temperature corrosion. To overcome these disadvantages several attempts have been made to employ thin corrugated metal sheet. The corrugations serve to support the plates against the pressure difference of the two streams. In U.S. Pat. No. 4,029,146, the corrugated metal sheets are mounted within a metal casing. The corrugated rims on two adjacent plates serve to separate the plates and form narrow channels through which the two fluids must flow. In many applications this arrangement presents the disadvantage that the narrow passages can become clogged by soot or other solid deposits, and differential thermal expansion between casing and plates also constitutes a problem. Other parallel plate heat exchangers are shown in U.S. Pat. Nos. 4,308,915, 1,727,124 and 2,368,814, United Kingdom Pat. Application No. 2,041,190A and France Pat. Application Nos. 75 20285 and 78 31863.